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Scheduling Algorithms for Modern Disk Drives

By Bruce L. \vorthington, Gregory R. Ganger and Yale N. Patt


Department of Electrical Engineering and Computer Science Disk subsystem performance can be dramatically im-proved by dynamically ordering, or scheduling, pending re-quests. Via strongly validated simulation, we examine the impact of complex logic & to-physicaJ mappings and large prefetching caches on scheduling effectiveness. Using both synthetic workloads and traces captured from six differ-ent user environments, we arrive at three main conclusions: (I) Incorporating complex mapping information into the scheduler provides only a marginal (less than 2YO) decrease in response times for seek-reducing algorithms. (2) Algorithms which effectively utilize prefetching disk caches provide sig-nificant performance improvements for workloads with read sequentiality. The cyclical scan algorithm (C-LOOK), which always schedules requests inascending logical order, achieves the highest performance among seek-reducing algorithms for such workloads. (3) Algorithms that reduce overall position-ing delays produce the highest performance provided that they recognize and exploit a prefetching cache.

Year: 2008
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